I would recommend a free antibody database available at labome.com. For antibodies to Foxp3 for IHC you can check the following link: https://www.labome.com/review/gene/human/FOXP3-antibody.html. eBioscience has mouse monoclonal (236A/E7) abs which are suitable for IHC on both human and mouse samples. Also, Abcam has rabbit polyclonal 54501 ab for IHC on both human and mouse samples. Santa Cruz Biotechnology also offers FOXP3 antibody (Santa Cruz, sc166212) for IHC (Crncec I et al., Methods Mol Biol. 2015). You can look at the references and the description of these and other Foxp3 antibodies following the links.